1. Field of the Invention
The present invention relates to a CDMA radio communication system and method in a digital radio communication using the CDMA (Code Division Multiple Access).
2. Description of the Related Art
FIG. 1 is a block diagram illustrating a configuration of a conventional CDMA radio communication system. In the CDMA radio communication system, at a transmission side, transmission data 1 is spread at spreading section 2. The spread signal is modulated at modulation section 3 and then amplified at transmission amplifying section 4. The resultant signal is transmitted from transmission antenna 5. An amplitude in transmission amplifier 4 is determined at high rate transmission power setting section 6.
On the other hand, at a reception side, a signal received at reception antenna is detected at detection section 12 and then despread at despreading section 13. Received data 14 is thus obtained. Then, received data 14 is subjected to estimation of slot (an interval of few symbols) quality at slot quality detection section 16. Based on the estimation result, high rate transmission power control section 15 calculates a control value of high rate transmission power. The control information is transmitted to high rate transmission power setting section 6 at the transmission side.
The signals spread at spreading section 2 are configured in order of symbol as illustrated in FIG. 2. FIG. 2 illustrates an example of the case of 8 symbols with spreading factor 16. A symbol is composed of 16 chips.
FIG. 3 is a diagram to explain the reception quality in the conventional CDMA radio communication system. FIG. 3 illustrates an example in which the order of transmitted chips differs between two users of user A and user B by 5 chips. In addition, fading in each link is independent.
With respect to signals of user A, since transmission power is controlled at high rate transmission power setting section 6 that performs the setting according to an instruction from high rate transmission power control section 15, the fluctuation of line quality due to fading is canceled. Hence, the quality of received data 14 shows an almost constant level as illustrated in FIG. 3. Thus, in the CDMA radio communication system, each user is controlled to transmit a signal in the required minimum transmission power according to the transmission power control, in order to decrease the total interference of the system and improve the system capacity.
FIG. 4 is a diagram illustrating an interference amount in the conventional CDMA radio communication system. As been understood from FIG. 4, MS transmission power varies in inverse proportion to the fading between MS and BS_A (hereinafter referred to as fading MS-BS_A. In other words, MS transmission power is adjusted to cancel fading MS-BS_A.
At this point, since the fading between MS and BS_B (hereinafter referred to as fading MS-BS_B) and fading MS-BS_A are each independent, the interference amount in BS_B that is an other base station varies in such a manner a large peak appears as illustrated in FIG. 4. Even in this case, the average value of interference amounts is less than the case where power transmission control in not performed. It is thereby possible to decrease the total transmission power in the system.
However, in the conventional CDMA radio communication system, since spread signals are arranged to be concentrated upon a successive short time in the same frequency, when the short time is affected by fading and shadowing, qualities of all chips in the time deteriorate at the same time, resulting in that the qualities are not improved even though the spreading gain is obtained by despreading. It thereby results in another problem that the system capacity is limited because the transmission is performed under large transmission power. In addition, in a CDMA-TDD system, since the common frequency is used in forward and reverse links, it is necessary, for example, for a base station to finish the reverse reception before starting the forward transmission. Therefore, a guard time corresponding to a propagation delay time is required with the length limited to ensure the transmission efficiency, thereby resulting in the problem that a cell radius is limited.
An object of the present invention is to provide a CDMA radio communication system and method which has resistances against fading and shadowing to improve qualities, and in a TDD system, enables a guard time to be reduced and a cell radius to be extended.
In the digital radio communication system, in the case where a temporarily long burst error is caused due to affects of, for example, fading, interleaving is performed to decrease the affects to be provided to an error correction decoding section by diffusing the burst error. This interleaving is performed for every slot, and therefore does not correspond to fading and shadowing.
Considering of the aforementioned point, the present inventors found out the interleaving of spread chips enables a quality of each symbol in a frame to be kept constant, while a rate of transmission power control to be alleviated, and achieved the present invention. It is thereby possible to reduce overhead and decrease interference amounts to other cells.